Meltblowing die having presettable air-gap and set-back and method of use thereof

ABSTRACT

A metblowing die designed to efficiently receive a preassembled die tip assembly is described. The die assembly includes a die body (19) mounted on a meltblowing machine and a replaceable die tip assembly (20). The die tip assembly (20) includes a mounting block (25), to which are mounted a die tip (26) and air knives (27, 28) on opposite sides or the die tip (26). The positions of the air knives (27, 28) are adjustable to set air gap and set back parameters important in proper operation. The die tip assembly (20) is moved into and out of the die body (19) after air knife adjustment permitting adjustment to be made with the die tip assembly (20) off the machine, allowing rapid replacement. The replacement die tip assembly (20) is preheated, permitting replacement while the die body (19) is still hot.

This application is a continuation in part of application Ser. No.07/835,190, filed on Feb. 2, 1992, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to meltblowing dies and specifically toa meltblowing die with a replaceable die tip, wherein the air-gap andset-back parameters may be adjusted as desired before replacement. Inanother aspect, the invention relates to a method of replacing the dietip of a meltblowing die.

2. Discussion of the Prior Art

Meltblowing is a process for manufacturing nonwoven products byextruding a molten thermoplastic polymer resin through a plurality ofsmall orifices to form fine fibers while blowing converging sheets ofair onto each side of the orifices to attenuate and draw down thefibers. The extruded fibers are blown onto a moving collector surfacewhere they entangle in a random way to produce a nonwoven fabric or web.The newly formed web is directly withdrawn from the collector forfurther processing as desired. The overall process is carried outcontinuously. The webs may be further processed into a number ofcommercially important products such as filters, fabrics, and batteryseparators among others. Meltblowing dies are disclosed in U.S. Pat.Nos. 3,978,185, 4,818,463, and 4,986,743.

A key component of a meltblowing die is the die tip. The die tip is anelongate member having an outwardly protruding nosepiece of triangularcross-section terminating in an apex with a row of side-by-side orificesdrilled through the apex. A polymer melt is forced through the orificesfor extruding the polymer and discharged as side-by-side molten orsemimolten fibers. The die tip is generally formed of high quality steelin a separate machining process and fastened to the face of the die bodyusing bolts. Precise machining is required to achieve uniform polymerflow over the length of the row of orifices. The polymer is forcedthrough the die tip orifices by applying very substantial pressure tothe molten polymer inside the tip. The internal pressure induces tensilestress in the die tip which tends to concentrate near the apex of thetip and may cause the tip to fail in tension. U.S. Pat. No. 4,986,743teaches a method for mounting the die tip on the die body with inducedcompression in the apex region to counterbalance the tensile stress,rendering the die tip more reliable in service. This patent isincorporated herein by reference.

Other important components of the meltblowing die are elongate platesreferred to as air knives which, in combination with the die tipnosepiece, form converging air flows to attenuate and draw down theextruded fibers to microsized diameters. The air knives are generallythick elongate plates which have a longitudinal edge tapered to form aknife edge. Conventionally, two air knives are bolted to the die body oneither side of the triangular nosepiece of the die tip. The taperededges of the air knives are aligned with the confronting taperedsurfaces of the nosepiece and spaced slightly therefrom to form two airflow channels converging at the apex of the nosepiece, so that the airflowing rapidly past the apex of the nosepiece tends to entrain thefibers being extruded therefrom, and draw the fibers down until theybreak. The air flow characteristics are determined by the shape and sizeof the passages formed between the die tip and the air knives, which aredefined by parameters known in the art as the "air-gap", the spacingbetween the confronting surfaces of the triangular nosepiece of the dietip and the air knives, and the "set-back", the vertical distancebetween the tip of the nosepiece and the outer plane of the air knives.(As used herein, terminology such as "upward", downward", "vertical",and the like refers to the usual orientation of the die, in which thefibers are extruded downwardly; however, the invention is not to be solimited).

The air-gap and set-back determine the geometry of the converging airflow passages, and thereby influence the air flow properties and thedegree of fiber-air interaction. Research has shown that controlling theair flow properties in relation to the polymer flow properties isimportant for achieving the desired degree of fiber attenuation andfinal fiber diameter. Research has further shown that the fiber diameterstrongly influences the properties of the web such as filtrationefficiency.

In typical die designs, the air-gap and set-back are adjustable forselecting the air flow characteristics, to obtain the desired fiber andweb product. Meltblowing polymers with different compositions mayrequire different air-gap and set-back. Large meltblowing dies arenormally vertically oriented and discharge downwardly onto a movingcollector surface such as a conveyor screen or rotating drum.

A common problem in meltblowing occurs when the die tip plugs orstructurally fails requiring that the die tip be removed for cleaning orreplacement. In a conventional meltblowing die, the die tip and airknives are separate components individually bolted to the face of a diebody fixed to the extruding and air supply components. Typicalmeltblowing die structures are shown in U.S. Pat. Nos. 4,818,463 and4,986,743. Removal of the die tip generally requires first that each airknife be detached from the die body to gain access to the die tip. Inorder to remove the air knives, the air knife bolts are removed whilethe air knives are supported by workers, by cranes or the like. When thebolts have been removed, the air knives may be lowered from the die faceand withdrawn from the workspace. The die tip bolts are then removedwhile the die tip is similarly supported, and the die tip is lowered andwithdrawn. For large dies this procedure may be hazardous due to thelarge weight of the air knives and die tip. For dies employing a largeconveyor-type collector, it may not be feasible to move either the dieor collector to facilitate replacement of a fouled or damaged die tip;this further complicates the procedure due to the restricted spacebetween the die and collector, forcing the workers to disassemble theair knives and die tip while beneath the die, in some cases beingobliged to stand on the collector. These difficult working conditionsare further exacerbated by high temperature of the die assembly. Morespecifically, in typical operation, the die tip and air knives may be attemperatures above four hundred degrees Fahrenheit and the meltblowingline must be shut down for an extended period to allow for coolingbefore repairs can be made or die tip replacement effected.

Similar difficulties are encountered in the reverse installationprocedure, which is further complicated by the necessity of adjustmentof the air-gap and set-back after the die tip and air knives have beenreattached. If conventional dies are employed, these adjustments must becarried out with the die tip and air knives mounted on the die body.Moreover, the adjustment process involves the steps of mounting the airknives in a first position with respect to the die tip, measurement ofthe air-gap and set-back, removal of the air knives and reassembly withthe air knives at different positions, remeasurement of the air-gap andset-back, and like tedious and repetitive operations, all commonlycarried out under very inconvenient working conditions. The result isthat often the adjustments are hurried and not made correctly.

SUMMARY OF THE INVENTION

The meltblowing die constructed according to the present inventionsimplifies the removal and installation of the meltblowing die tip andcircumvents many of the difficulties encountered as noted above in useof conventional dies. The invention involves the use of a meltblowingdie body designed to efficiently receive a preassembled, preadjusted dietip assembly including a die tip and air knives assembled to a mountingblock. The invention may be applied to dies of any size, but is ofparticular utility for larger dies (e.g., those of fifty inches orlonger).

The die tip assembly of the present invention comprises a meltblowingdie tip having a triangular nosepiece and air knives mounted on amounting block. The mounting block with the die tip and air knivesmounted thereon in preadjusted relationship is then assembled to a diebody. The die body and mounting block include mating passages for themolten polymer and for compressed air, such that the passages are sealedto one another when the mounting block is assembled to the die body.

In a particular embodiment of the invention, the die body defines anopen-ended elongate cavity for receiving the preassembled die tipassembly. The die tip assembly is guided into position alonglongitudinal intermeshing shoulders (e.g., rails or ribs) formed alongthe cavity walls. In vertically oriented dies, the longitudinalintermeshing shoulders support the weight of the die tip assembly as itslides lengthwise into position within the cavity. In another embodimentof the invention, the die tip assembly is inserted into the cavity inthe die body from beneath.

The die tip assembly is secured to the die body by applied forcescompressing the back side of the die tip assembly against theconfronting back wall of the die body cavity, whereby effectivemetal-to-metal seals are formed, sealing the mating polymer and airpassages to one another. The forces may be applied by threaded bolts orhydraulically activated pistons. Where the die body includes ribsreceived by grooves in the mounting block, as in the particularembodiment mentioned above, the securing forces may be removed withoutdanger since the weight of the die tip assembly is supported entirely bythe ribs of the die cavity. The compressive forces acting on the die tipassembly thus serve to secure the assembly to the die body and to form afluid seal between the assembly and the die body.

As noted, according to an important aspect of the present invention, thedie tip and the air knives are preassembled into their selectedrespective operating positions on the mounting block before the die tipassembly is installed in the die body. The preassembly step may beaccomplished, including pre-setting of the air-gap and set-back topredetermined values, without repetitive adjustment steps. The die tipassembly components will generally be preassembled on a planar worksurface and then transported to the die using a cart or crane; in theparticular embodiment discussed above, the die tip assembly is thenaligned with and slid into the die body cavity. In this embodiment ofthe invention, the replacement die assembly can be used to push anexisting assembly through the opposite open end of the die body cavityfor removal.

Because according to the method of the present invention, the die tipassembly is completely adjusted before it is assembled to the die body,the die tip assembly may be preheated to near the meltblowingtemperature for direct insertion into a hot die. Preheating the die tipassembly induces thermal expansion in the die tip assembly approximatelyequal to that in the die body, so that the mating flow passages andother components of the die tip assembly and the die body alignproperly. Preheating the die tip assembly also prevents thermal stressand buckling or warping of the die tip assembly, which might occur if acold die tip assembly were inserted into a heated die body.

A specifically designed cart may be employed for efficientlytransporting, preheating, installing and removing the die tip assembly.The cart may be equipped with a furnace enclosure for preheating theassembly, an alignment device for aligning the die tip assembly with thedie body, and a hydraulic ram or the like for forcing the die tipassembly into or out of the die body. The cart may also include anassembly support surface comprising a roller table or the like foreasily delivering and receiving the die tip assembly.

The present invention simplifies the procedure of replacing a fouled,worn or damaged die tip to the point that a hot die tip assembly may beremoved and a preheated replacement die tip assembly installed, that is,eliminating the cooling period required before replacing a conventionaldie, and thus reducing the time required to replace a die tip anddecreasing the possibility of personal injury or damaging the die tipduring installation or removal. The die tip assembly is preferablysecured to the die body using quick release fasteners, for example boltswhich need merely be loosened to release the die tip assembly, orhydraulically activated pistons. For replacing a fouled or damaged dietip, the line is momentarily shut down and the old die tip assemblyfreed. A previously adjusted die tip assembly is then preheated, andslid into the die, displacing the old assembly for removal. The newassembly is secured for operation and the line restarted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the main components of ameltblowing resin processing production line, including the meltblowingdie assembly constructed according to the present invention.

FIG. 2 is a cross-sectional view illustrating details of the die body.

FIG. 3 is a cross-sectional view illustrating details of the die tipassembly.

FIG. 4 is a cross-sectional view illustrating the die tip assemblymounted in the die body.

FIG. 5 is a cross-sectional view taken along the line 5--5 in FIG. 3,illustrating details of the die tip assembly mounting block.

FIG. 6 is a cross-sectional view taken along the line 6--6 in FIG. 3,illustrating details of the assembly spacer plate.

FIG. 7 is a cross-sectional view taken along the line 7--7 in FIG. 3,illustrating details of the air knife assembly.

FIG. 8 is a dimensioned side view of the die tip assembly for definingthe set-back.

FIG. 9 is a side elevational schematic diagram illustrating ameltblowing polymer processing line having a vertical die, together witha cart carrying a replacement die tip assembly.

FIG. 10 is a view comparable to FIG. 2, illustrating the die body in analternative embodiment of the invention.

FIG. 11 is a view comparable to FIG. 3, illustrating the die tipassembly in the alternative embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, the main components of a meltblowing polymerprocessing line include an extruder 10, a meltblowing die assembly 11,and a moving collector screen 15. The extruder 10 supplies moltenpolymer to die assembly 11 which discharges a plurality of side-by-sidemolten or semimolten fibers into converging air sheets to draw down andattenuate the fibers, forming a fiber/air stream 12. The air is suppliedthrough valved lines 13, and heating elements 14 are provided in the die11 for heating the polymer. The fiber/air stream 12 flows downwardlyonto the moving collector screen 15 where the fibers tend to entangleupon deposit, to form a nonwoven web 16.

The meltblowing die assembly 11 constructed according to the presentinvention includes a die body 19 and a replaceable die tip assembly 20.The die assembly 11 as illustrated and described herein has a verticalorientation wherein the fibers are discharged downwardly onto anunderlying collector 15, as is typical. It is to be understood, however,that use of the terms "vertical", "downwardly", "upwardly" and the likeherein and in the claims are merely for reference and clarity inexplaining the invention, and are not to be construed as limiting terms.The meltblowing die assembly 11 can have horizontal or intermediateorientations as well.

The die body 19 and die tip assembly 20 are separate components asillustrated in FIGS. 2 and 3. FIG. 4 illustrates the two componentsassembled.

As shown by FIG. 2, the die body 19 has formed therein an elongatecavity 17 defined by downwardly facing bottom surface 18 and inwardlyfacing side walls 21a and 2lb. In a particular embodiment of theinvention described first, longitudinally extending ribs 22 and 23protrude outwardly from the walls 21a and 2lb, respectively, and serveto support the die tip assembly 20 as described below. (In a furtherembodiment of the invention described with reference to FIGS. 10 and 11,ribs 22 and 23 are eliminated. Where not indicated otherwise, these twoembodiments are substantially similar.)

The die body 19 has formed therein a polymer flow passage 24 dischargingat bottom wall 18. The polymer flow passage 24 may be in the form of acoat hanger die as described in U.S. Pat. No. 4,818,463, and is providedwith channel 24a extending along the length of surface 18.

As shown in FIGS. 3 and 4, die tip assembly 20 comprises mounting block25, die tip 26, and air knives or air plates 27 and 28. Mounting block25 is preferably in the form of a U-shaped member having a groove 29formed therein. Die tip 26 is received on raised mounting surfaces 31aand 3lb formed on either side of the groove 29. Grooves 32 and 33 formedin opposite sides of mounting block 25 are sized to engage ribs 22 and23, respectively, of the die body. The block 25 is provided withupwardly facing surface 42.

Referring to FIG. 4, mounting block 25 fits in close conformity incavity 17 in die body 19, such that the die tip assembly 20 can be movedslidingly lengthwise therein, with elongate die body ribs 22 and 23engaging mounting block grooves 32 and 33, respectively. The slidingmotion of assembly 20 with respect to die body 19 occurs into or out ofthe plane of FIG. 4. Die body cavity 17 is open at both ends so that dietip assembly 20 may slide into or out of the cavity from either side ofdie body 19. The present invention contemplates any orientation of thedie, although the vertical orientation is preferred. For verticalorientation of the die body 19 (as in FIG. 1), die body ribs 22 and 23within groove surfaces 32a and 33a, respectively, support the die tipassembly 20.

The die tip assembly 20 is secured to die body 19 by bolts 34 and 35which act upon mounting block 25 to apply compressive forces urging theupper surface 42 of the mounting block 25 into firm engagement with thelower surface of die body 19. Bolts 34 and 35 are threaded within angledholes 36 and 37 in die body 19. Beveled surfaces 38 and 39 on die body19 are provided for clearance, but do not frictionally engage the headsof bolts 34 and 35, respectively. The distal ends of bolts 34 and 35engage longitudinally-extending angled grooves 40 and 41, respectively,formed in opposite sides of mounting block 25 and extending parallel togrooves 32 and 33. Grooves 40 and 41 terminate in flat surfaces 40a and41a. Accordingly, upon tightening of bolts 34 and 35, an axial upwardforce is imparted to surfaces 40a and 41a, compressing the upper surface42 of the assembly mounting block 25 against the confronting bottomsurface 18 of die body cavity 17. The compressive force so exerted issufficient to secure die tip assembly 20 to die body 19 for operation,and to seal mating polymer and air flow passages to one another. Thebolts 34 and 35 thus serve as compression rods for applying compressiveforces to secure the die tip assembly 20 to the die body 19.Equivalently, hydraulic pistons can be used as compression rods to applythe compressive forces.

The dimensions of grooves 32 and 33 in relation to ribs 22 and 23,respectively, are Such as to allow the mounting block to move upwardlyand engage the die body upon tightening of bolts 34 and 35, and to allowthe die tip assembly to slide along ribs 22 and 23 upon loosening ofbolts 34 and 35. Thus, to allow sliding installation or removal of theassembly 20, bolts 34 and 35 need merely be loosened, and need not befully withdrawn. The weight of die tip assembly 20 is supported on diebody ribs 22 and 23.

In practice, multiple bolts 34 and 35 will be spaced along the length ofthe die, the number of bolts depending on the die length. To facilitatecentering the die tip assembly 20 in the die body 19, one of thethreaded holes 36 may be provided at the centerline of die body 19 foralignment with a tapered centering hole 43 (see FIG. 5) formed insurface 40a of mounting block 25 for receiving a tapered centering bolt(not shown).

As indicated, die body 19 and mounting block 25 include mating flowpassages for molten polymer and for compressed air. More specifically,die body polymer flow passage 24 is in fluid communication with elongateflow passage 45 formed in mounting block 25. Similarly, air passages 51in the die body 19 mate with air passages 52 and 53 in the mountingblock 25.

As mentioned, securing bolts 34 and 35 exert sufficient compressiveforce to form a metal-on-metal fluid seal between machined planarsurfaces 18 of the die body 19 and 42 of the mounting block 25 andestablish a fluid seal around the Junction of polymer flow channel 24aand flow passage 45. Recesses 46 and 47 are provided in surface 18 todecrease the contact area between surfaces 42 and 18, and therebyincrease and concentrate the compressive stress in the region around thepolymer flow passage junction for forming the seal. Although notessential, O-ring 48 may be provided around the Junction to furtheraccomplish the seal.

Mounting block flow passage 45 receives a polymer melt from die bodychannel 24a and conducts the molten polymer to flow passage 49 formed indie tip 26. Flow passage 45 extends substantially the length of mountingblock 25 and may be longitudinally continuous or may comprise an in-lineseries of shorter elongate passages closely spaced over the length ofthe mounting block, for improved mechanical strength. For long dies, thelatter configuration is preferred, to increase the structural integrityof the mounting block. The end enclosures of polymer passage 45 areformed by end walls 77 and 78 (see FIGS. 1 and 9) of mounting block 25,while end enclosures of polymer passage 49 are defined by end walls ofdie tip 26. The outer periphery of end plates 77 and 78 are sized toallow assembly 20 to be slidingly moved into or out of die body cavity17 with the end plates attached, such that end plates 77 and 78 areconsidered to be components of die tip assembly 20.

Air flow passages 51 formed on opposite sides of die body 19 registerwith air passages 52 and 53, respectively, formed in mounting block 25.The compression of assembly surface 42 onto surface 18 by securing bolts34 and 35 establishes metal-to-metal fluid seals at the Junctions of theair flow passages, as has been described in relation to polymer flowpassages 44 and 45.

Die tip 26 and air knives 27 and 28 define converging air channels 75and 76. Air supplied through passages 51 in the die block 19 flowsthrough passages 52 and 53 in the mounting block 25 and then throughpassages 54 and 55, respectively, formed between the air knives 27 and28 and the die tip 26. The air flow passages 52 and 53 are preferably inthe form of a plurality of holes spaced along the length of the mountingblock 25 to provide uniform distribution of air along the die tip 26,and register with a like number of air passages 51 in the die body 19.Air deflector plates 56 and 57 are provided to induce mixing of airdischarged from the plurality of air passages 52 and 53, and therebyachieve a uniform air flow along the length of air passages 54 and 55,respectively. Deflector plates 56 and 57 may be secured to flanges 59and 60 of the die tip 26 by bolts (not shown).

The die tip assembly of the present invention preferably employs the dietip mounting technique disclosed in U.S. Pat. No. 4,986,743(incorporated herein by reference). Although this die tip mountingtechnique is preferred, other die tip mounting techniques may also beused without departing from the present invention.

As best seen in FIGS. 3 and 4, die tip 26 is an elongate member having aprotruding triangular nosepiece 58 and flanking flanges 59 and 60.Clearance holes 61 and 62 in the flanges 59 and 60 are counterbored at61a and 62a at one end, and aligned with threaded holes 63 and 64,respectively, in the mounting block 25 to receive bolts 65 and 66,securing die tip 26 to mounting block 25. Cover plates 67 and 68 coverthe counterbored holes 61 and 62, and are attached to die tip flanges 59and 60 using bolts (not shown).

Polymer flow passage 49 formed in die tip 26 is in fluid communicationwith mounting block flow passage 45. The polymer flow passage extendsthrough flow distribution plate or "breaker plate" 69. Breaker plate 69contains a plurality of smaller holes 69a which serve to distribute thepolymer flow uniformly over the length of the die tip flow passage 49.On one side, breaker plate 69 fits in close conformity in a groove 74formed in die tip 26. On the opposite side, breaker plate 69 confrontsthe mounting block 25 along a portion of surface 30. As taught by U.S.Pat. No. 4,986,743, the dimensions of breaker plate 69 in relation togroove 74 are such that as the die tip mounting bolts 65 and 66 aretightened, metal-on-metal fluid seals are established between breakerplate 69 and confronting surface 30 on one side of the breaker plate,and between breaker plate 69 and the confronting bottom surface 73 ofgroove 74 on the other side, thereby establishing a fluid seal extendingbetween polymer flow passages 45 and 49.

Die tip nosepiece 58 protrudes outwardly and terminates at apex 71. Oneor more rows of equally spaced orifices 72 are drilled through the apex.The spacing of the orifices is usually between 10 to 40 orifices perinch and the orifice diameters are generally between 0.010 to 0.025inches. Accordingly, the pressurized polymer melt flows through passages44 and 45, through breaker plate 69, into die tip passage 49, andthrough orifices 72, being discharged as a plurality of side-by-sidemolten or semimolten fibers.

The internal pressure in polymer flow passage 49 tends to induce atensile stress in the tip which concentrates at the apex 71 of thenosepiece 58 and may cause the tip to fail in tension. Mounting of thedie tip in accordance with U.S. Pat. No. 4,986,743 applies a compressivestress in the apex region of the nosepiece which counterbalances thetensile stress, thereby reducing the resultant operating stress in thenosepiece, and improving its reliability in use.

Thus, in accordance with the teachings of U.S. Pat. No. 4,986,743,breaker plate 69 is fully received in groove 74 and die tip 26 ispositioned on raised shoulders 31a and 31b on the mounting block 25. Theupper surface 73 of breaker plate 69 confronts the surface 30 ofmounting block 25 surrounding flow cavity 45. With the die tip 26positioned on the shoulders 31a and 31b, but not firmly bolted in place,the breaker plate 69 and die tip 26 are both spaced from mounting block25. Upon tightening bolts 65 and 66 to the fully stressed position, thedie tip 26 undergoes a small inward deflection, with shoulders 31a and31b acting as fulcrums. The spacing between surface 73 of breaker plate69 and surface 30 of mounting block 25 is reduced to zero as thesurfaces are interfaced in tight compression, while the spacing 70between surface 74 of the die tip 26 and surface 30 of the mountingblock 25 remains greater than zero. The compression of the surface 73 ofbreaker plate 69 onto 30 seals polymer flow passages 45 and 49 asdescribed above. The inward deflection of the die tip 26 induces acompressive bending moment concentrated in the apex region of thenosepiece 58. The resultant compressive stress in the apexcounterbalances, in part, the tensile stress induced by internalpressure in flow passage 49 during operation.

Employment of the teachings of U.S. Pat. No. 4,986,743 as thus describedrequires that the die tip 26 be formed as a separate member bolted tomounting block 25. While as indicated this construction technique ispreferred, it is within the scope of the present invention to form thedie tip integrally with the mounting block 25.

Air knives 27 and 28 are elongate plates with inner surfaces 27b and28b, respectively, contoured in general conformity with confrontingsurfaces of die tip 26 and spaced therefrom to form flanking air flowpassages 54 and 55, respectively. Air knives 27 and 28 have inwardlyprojecting flanges 27a and 28a which terminate in spaced apart edges 79,and, in combination with confronting die tip nosepiece 58, formconverging air flow passages 75 and 76. The width of passages 75 and 76,that is, the spacing of air knife edges 79 from the apex 71 of thenosepiece, is the parameter known to the art as the "air-gap". Airsupplied on opposite sides of the die 11 via lines 13 flows through airchambers 50 (see FIG. 1), through passages 51, passages 52 and 53,passages 54 and 55, and into converging air flow passages 75 and 76. Theangle of convergence of the airstreams flowing from passages 75 and 76is defined by the internal angle of nosepiece 58, and is typicallybetween 45 to 90 degrees. Passages 75 and 76 discharge converging airsheets onto opposite sides of die tip orifices 72 to attenuate and drawdown the fibers issuing therefrom and to form a fiber/air stream 12.

The positions of air knives 27 and 28 in relation to die tip 26 areadjustable vertically and horizontally for controlling the flowproperties of the converging air sheets discharging from flow passages75 and 76. The air flow properties are determined by the air-gap,dimension E in FIG. 8, and the set-back, dimension F, which parametersin turn are determined by the positions of the air knives 27 and 28 withrespect to the nosepiece of die tip 26. As noted, the "air-gap"E is thewidth of air flow passages 75 and 76, while the "set-back"F is thevertical distance between the knife edges 79 of the air knives and apex71 of the die tip. Each air knife 27 and 28 will usually be positionedat the same air-gap and set-back on opposite sides of the apex. Theproper air-gap and set-back settings depend on the composition of thepolymer melt being processed, the desired web properties, the size ofthe die, and like process variables. For meltblowing polypropylene, theair-gap is typically between 0.010 to 0.100 inches and the set-back isbetween -0.020 to +0.100 inches. As mentioned above and described indetail below, according to an important aspect of the invention, theair-gap and set-back may be preset before the die tip assembly isassembled to the die body 19 for use.

According to one aspect of the invention, plural discrete selectibleoptions are provided for the vertical relation of the air knives 27 and28 to die tip 26, while the lateral position of air knives 27 and 28 isadjusted by selection of appropriate spacer plates 83, 87 between airplates 27 and 28 and mounting block 25. The air-gap and set-backparameters are functions of these relative vertical and lateralpositions. Conveniently, the user is supplied with a table which may beconsulted to determine the desired vertical position and spacer platethickness for various combinations of desired air-gap and set-backparameters. The selected air-gap and set-back parameters can then bepreset by appropriate assembly of the air plates to the mounting block,that is, before insertion of the die tip assembly 20 into cavity 17 indie body 19.

As shown in FIGS. 4 and 5, mounting block 25 is provided with aplurality of vertical slots 80 counterslotted at 80a to receive airknife securing bolts 82 threaded into holes 81 in air knives 27 and 28.For clarity, the configuration is shown for air knife 28 only. Air knife27 is provided with securing means of the same design. Air knife 28 isthus secured to mounting block 25 by bolts 82, and is spaced aselectible distance therefrom by spacer plate 83. Counterslots 80a areprovided so that the heads of bolts 82 will not interfere with thesliding of assembly 20 into die body cavity 17, and to allow verticaladjustment of the position of air plate 28 with respect to mountingblock 25. Spacer plate 83 is provided with aligned slots 83a for thepassage of bolts 82 therethrough. For any particular set-back of the airknife, the thickness of the spacer plate 83 determines the air-gapbetween the air knife and die tip nosepiece 58. Upon tightening bolts82, the air-gap is set positively since no further adjustments areneeded for operation. Spacer plates of specified thickness can beinterchanged to obtain the desired air-gap; as noted, according to animportant aspect of the invention, this procedure is completed prior tothe installation of the die tip assembly 20 in die body 19.

The set-back adjustment means is shown in FIGS. 4 through 7 for airknife 27 only; in practice air knife 28 will also be provided with aset-back adjustment means of the same design. Mounting block 25 has aplurality of sets of vertically and laterally spaced dowel pin holes84a-84c in alignment with a like number of sets of vertically alignedand laterally spaced dowel pin holes 85 in air knives 27 and 28 forreceiving dowel pins 86. See FIGS. 5-7. By inserting dowel pins 86 inappropriate ones of holes 84a-84c in block 25 and holes 85 in air knives27 and 28, the vertical positions of air knives 27 and 28 are fixed withrespect to block 25. Air-gap spacer plate 87 (identical to spacer plate83) is provided with sets of aligned slots 87a (see FIG. 6) to permitdowel pins 86 to pass therethrough. The length of dowel pins 86 is suchthat when fully inserted, the outer free ends of the pins 86 do notinterfere with the sliding of die tip assembly 20 into die body 19.Dowel pins 86 are provided with threaded holes 86a for receiving athreaded pull out means for withdrawing pins 86.

With the dowel pins 86 installed in the corresponding ones of holes84a-84c and 85a-85c, air knife 27 is spaced vertically from die tipnosepiece 58 by a desired set-back (dimension F in FIG. 8). Thus, upontightening air knife securing bolts 82, the set-back of the air knives27 and 28 is set positively; again, according to an important aspect ofthe invention, this procedure is accomplished before installing die tipassembly 20 in die body 19.

For the purpose of describing the adjustable set-back means of thepresent invention, dimensions G, H, and I are identified in FIG. 8 withreference to air knife 27. The set-back, F, is related to dimensions G,H, and I as follows: F=I-(H-G). According to the invention, G is fixed,in that the holes 85 bored in the air knives 27 and 28 are verticallyaligned, lying along a horizontal line, and I is fixed, in that theposition of the die tip 26 with respect to the mounting block 25 is notadjustable. For fixed values of G and I, the set-back of air knife 27can be varied by varying the dimension H (i.e., by choosing a desiredmounting block dowel pin hole 84). The present invention provides foradjustable set-back by providing sets of dowel pin holes 85a-85c drilledin the air knives 27 and 28 at a fixed value of dimension G, and a likenumber of sets of dowel pin holes 84a-84c drilled in mounting block 25at differing predetermined values of the dimension H above a referenceline. For setting the desired set-back, for example, of air knife 27,the air knife is moved vertically until one of the air knife holes85a-85c in each set thereof is aligned with the desired one of themounting block holes 84a-84c in each set thereof, and the dowel pins 86are inserted therethrough. Thus, dowel pin holes 84 as illustrated inFIGS. 4 and 8 represent ones of a number of sets of selectible mountingblock dowel pin holes 84a-84c, each hole determining a different valueof the dimension H.

FIGS. 5, 6 and 7 illustrate the details of the adjustable set-backmeans, and the interrelationship between the mounting block 25 (FIG. 5),the spacer plate 87 (FIG. 6), and the air knife 27 (FIG. 7). The cuttingplane of each Figure is indicated in FIG. 3 as planes 5--5, 6--6, and7--7, respectively. Mounting block 25 is provided with multiple dowelpin holes 84a, 84b, and 84c, which are grouped on the basis of theircenterline distances Ha, Hb, and Hc, respectively, from the lowersurface of mounting block 88. The holes 84a-84c are arranged in arepeated pattern along the length of the mounting block; the number ofholes in each set will depend on the number of desired set-backpositions to be provided, while the number of sets of holes will dependon the length of the die. The holes are staggered laterally over thelength of the die, as opposed to being drilled in-line vertically (i.e.along lines perpendicular to lower surface of block 88), because thevertical spacing of the holes (e.g., Ha minus Hb) is small relative tothe diameters of the holes themselves. FIG. 5 illustrates a die tipassembly with three allowable set-back settings, each settingcorresponding to a particular group of dowel pin holes. More possibleset-back values can be provided in the same general way. Thus, it can beseen that a plurality of selectible discrete vertical positions of theair plates with respect to the nosepiece are defined by a plurality ofsets of vertically and laterally spaced holes 84a-84c in the mountingblock cooperating with a like plurality of sets of vertically alignedand laterally spaced holes 85a-85c in the air plates.

Air knife 27 has dowel pin holes 85a, 85b and 85c which are grouped onthe basis of their longitudinal alignment with mounting block dowel pinholes 84a, 84b, and 84c, respectively. The air knife holes 85a, 85b and85c are vertically aligned, that is, are spaced along a line parallel tothe lower planar surface of the air knife 89 and spaced a distance Gtherefrom. Due to the staggered vertical position of the mounting blockdowel pin holes 84a-84c , it is not possible for more than one group ofthe air knife holes 85a-85c to be simultaneously aligned with itsrespective group of mounting block holes 84a-84c over the entire lengthof the assembly. Thus for receiving the dowel pins 86, only one of thefollowing combinations is possible: holes 85a will align with holes 84a,holes 85b will align with holes 84b, or holes 85c will align with holes84c. Once a particular alignment is made and the dowel pins insertedtherein, the set-back is determined. The mounting block holes and theair knife holes are drilled with low clearance for receiving the dowelpins snugly, ensuring good alignment over the length of the assembly.The variation In the air-gap and set-back over the die length isgenerally less than ±0.002 inches.

If dowel pins 86 are removed and bolts 82 loosened, air knife 27 can bemoved vertically with bolts 82 sliding along slots 80, while bolts 82maintain longitudinal alignment of the mounting block and air knife. Inthis way, the air knife can be moved until the desired group of mountingblock dowel pin holes 84a-84c (corresponding to the desired set-back)aligns with the respective group of air knife holes 85a-85c, and thedowel pins 86 may then be inserted therein. Securing bolts 82 aretightened to positively set the set-back and secure the air knife foroperation. Spacer plate 87 has multiple slots 83a and 87a in alignmentwith the bolt holes and dowel pin holes for allowing the bolts and dowelpins to pass therethrough, respectively.

Referring to FIG. 8, it can be seen that for any particular thickness ofspacer plate 87, decreasing the set-back, F, will also decrease theair-gap, E, while increasing the set-back increases the air-gap.Therefore, for a particular set-back, there is a unique spacer thicknesswhich will provide the desired air-gap. For each of the allowableset-back settings, the variation in air-gap with respect to the spacerplate thickness is known from the engineering design analysis. The datarelating various dowel pin locations and shim thickness values tovarious combinations of air-gap and set-back parameters can be suppliedto the user in tabular form to simplify setting of the desired air-gapand set-back. Notice further that the set-back position is determinedcompletely by selection of mounting block dowel pin holes occupied bythe dowel pins, so that the set-back can be visually verified, withoutphysical measurements.

FIG. 9 illustrates a vertically oriented die body 19 prepared to receivethe die tip assembly 20 of the present invention. The die tip assembly20 will generally be preassembled on a planar work surface with theair-gap and set-back set to desired values, as described. The assemblywill then be transported to the meltblowing processing line forinstallation. To facilitate this operation, a specially designed cart 90may be provided, comprising an aligning guide for aligning the die tipassembly 20 with die body cavity 17, heating enclosure 92 (such as afurnace), ram 93, and roller surface 94. The aligning guide may comprisealigning rods 91 mating with aligning holes drilled in the end wall ofthe die body 19. For efficiently replacing a fouled or damaged die tipassembly, the line will be momentarily shut down and bolts 34 and 35will be loosened but need not be fully removed to free the die tipassembly to be replaced. The angular orientation of bolts 34 and 35improves accessibility thereto. The air-gap and set-back of thereplacement die tip assembly 20 are preset to provide the preferredairflow characteristics. The replacement die tip assembly is preheatedto a temperature comparable to that of the die tip assembly to bereplaced, typically while oriented face-down on roller surface 94.Aligning rods 91 are then inserted into the die body alignment holes,such that grooves 32 and 33 formed in the mounting body 25 are alignedfor engaging die body ribs 22 and 23 as has been described. Ram 93 isemployed for sliding assembly 20 into the cavity of die body 19; rollersurface 94 facilitates delivering the assembly from the cart 90 to thedie body 19. Heating enclosure 92 may comprise a gas or electric furnacefor preheating the assembly 20 before direct insertion into a hot diebody 19. For receiving the die assembly being displaced through theopposite end of the die cavity as has been described, a similar cart 90would be placed on the opposite side of the die 11. The new die tipassembly is then secured to the die body to form a fluid sealtherebetween as has been described. The line will be restarted andproduction resumed. To further simplify the replacement procedure, bolts34 and 35 may be replaced with quick release connectors such ashydraulically activated pistons. The pistons can be deactivatedsimultaneously to free the old assembly and reactivated simultaneouslyfor securing the new assembly to the body and for forming the fluidseals.

A problem in the replacement procedure may arise due to the dribbling ofmolten polymer from die body flow passage 24 after the line has beenshut down. Any residual material which lodges between sealing surfaces42 and 18 will resist the compression of the mating surfaces upontightening of bolts 34 and 35, and may result in failure of the sealtherebetween. Therefore, it would be undesirable for any polymerdribbling from die body 44 to flow into the space separating surfaces 42and 18 as a replacement assembly slides into position. This possibilityis precluded by the present invention; grooves 32 and 33 in the mountingblock 25 engage ribs 22 and 23 such that the end wall of the mountingblock 25 is Juxtaposed to the end wall of the die body as the die tipassembly slides into the die body, effectively "shaving" off anydripping polymer. As the die tip assembly slides further into position,passage 45 is in general alignment with polymer flow passage 44, so thatsubstantially any polymer dribbling from passage 44 will flow intopassage 45, and will not be collected on the mating surfaces of themounting block 25 and die body 19. A small amount of polymer dribblinginto passage 45 is acceptable when executing a normal assemblyreplacement procedure.

FIGS. 10 and 11 illustrate a further embodiment of the invention,wherein a replacement die assembly is lifted into the cavity 17 in thedie body from beneath, rather than being slid in from one end, as in theembodiment described above. Accordingly, the ribs 22 and 23 in the diebody cavity 17 and the cooperating grooves 32 and 33 in the mountingblock 25 are eliminated. In the embodiment of FIGS. 10 and 11, themounting block 25, having had die tip 26 and air knives 27 and 28assembled thereon in preadjusted relationship to one another to presetthe desired air-gap and set-back parameters, is lifted into cavity 17and secured therein by mounting bolts 96 extending through bores 97 inmounting block 25 and threaded into die body 19. As above, bolts 96 thussecure the die tip assembly within the die body 19 and ensure theintegrity of the air and polymer passages formed therebetween.

It is anticipated that the embodiment of FIGS. 10 and 11 may find usewhere the collector 15 may be readily moved from beneath the die body;under these circumstances, it may be more convenient to lift thereplacement die tip assembly into the die body from beneath than toslide it into place from one end thereof.

The die assembly according to the present invention is used in practiceof a corresponding method of the invention, that is, rapid and efficientreplacement of a die tip, while permitting the important air-gap andset-back parameters to be set and allowing the die tip assembly to beheated near operating temperature prior to assembly of the die tip andair knives to the die body. Similarly, a method of forming a meltblownpolymer product comprises the steps of:

(a) extruding a molten polymer through the orifices of the die tip at ameltblowing operating temperature;

(b) adjusting a replacement die tip to provide desired air-gap andset-back values;

(c) preheating the replacement die tip to substantially the sametemperature (e.g., within 10 degrees Fahrenheit) as the die tip assemblyto be replaced;

(d) discontinuing the extrusion of the molten polymer;

(e) moving the die tip out of the die assembly;

(f) moving the replacement die tip into the die assembly; and

(g) recommencing extrusion of the polymer.

Die tip replacement according to the invention may be carried out atelevated temperatures (e.g., 400 degrees Fahrenheit and above) withoutwaiting for the die to cool down to room temperature. Even at 250degrees Fahrenheit thermal expansion resulting from inserting a die tipat room temperature could present problems. The problem of thermalexpansion, however, is more serious at 350 degrees Fahrenheit and above.It is preferred to preheat the die to within fifty degrees, andpreferably 25 degrees of the operation temperature. It is also preferredto replace the die before it has cooled more than 50 degrees Fahrenheit.

In a preferred method, the die tip is replaced while maintaining the dieassembly at operating temperature, that is, the die heaters 14 are notshut off during die tip replacement.

Thermoplastic materials suitable for the process of the inventioninclude any thermoplastic useable in meltblowing. These includepolyolefins including homopolymers, copolymers (e.g., EVA), terpolymers,etc. Other suitable materials include polyesters such aspoly(methylmethacrylate) and poly(ethylene terephthate); polyamides suchas poly(hexamethylene adipamide), and poly(omega-caproamide), andpoly(hexamethylene sebacamide); polyvinyls such as polystyrene. Otherpolymers may also be used, such as nylon, polytrifluorochloroethylene,polyurethanes, polycarbonates, silicones, pitch, and blends thereof. Thepolyolefins are preferred. These include homopolymers and copolymers ofthe families of polypropylenes, polyethylenes, and other highpolyolefins. The polyethylenes include LDPE, HDPE, LLDPE, and very lowdensity polyethylene.

A broad range of process conditions may be used according to the processof the invention depending upon the thermoplastic material chosen andthe type of web/product properties needed. Any operating temperature ofthe thermoplastic material is acceptable so long as the material isextruded from the die so as to form a nonwoven product. An acceptablerange of temperature for the thermoplastic material in the die, andconsequently the approximate temperature of the die head around thematerial, is 350 degrees Fahrenheit to 900 degrees Fahrenheit. Apreferred range is 400 degrees Fahrenheit to 750 degrees Fahrenheit. Forpolypropylene, a highly preferred range is 400 degrees Fahrenheit to 650degrees Fahrenheit.

Any operating temperature of the air in the air knife is acceptable solong as it permits production of useable nonwoven product. An acceptablerange is 350 degrees Fahrenheit to 900 degrees Fahrenheit.

The flow rates of thermoplastic and air may vary greatly depending onthe thermoplastic material extruded, the distance of the extrusion headfrom the collector device, and the temperatures employed. An acceptablerange of the ratio of pounds of air to pounds of polymer is about20-500, more commonly 30-100 for polypropylene. Typical polymer flowrates vary from about 0.3-1.5 grams/hole/minute, preferably about0.5-1.0.

While several preferred embodiments of the invention have been describedin detail, these are not to be considered limiting on the invention,which is limited only by the following claims.

What is claimed is:
 1. A meltblowing process including the step ofreplacing a die tip assembly in use with a preadjusted die tip assembly,comprising the steps of:providing a meltblowing machine, comprising inuse:(i) an elongated die body having a coathanger-shaped polymer flowpassage formed therein, an upper inlet end of said coathanger-shapedpolymer flow passage being connected to a source of polymer to bemeltblown, and a lower discharge end of said coathanger-shaped polymerflow passage forming an elongate polymer flow port in a lower surface ofsaid die body, said die body further having air flow passages disposedon either side of said polymer flow passage, and said die body beingadapted to receive heaters for heating said polymer and air, and (ii) adie tip assembly, the machine further comprising a replacement die tipassembly, said die tip assembly in use and said replacement die tipassembly each comprising:(a) an elongated mounting block having an uppersurface for mating with and being secured to said lower surface of saiddie body, such that polymer and air flow passages formed in saidmounting block mate with said polymer and air flow passages formed insaid die body, (b) a die tip mounted to the mounting block, having adownwardly extending elongate nosepiece of generally triangularcross-section terminating at an apex, an elongated polymer channelwithin said die tip,and a plurality of small-diameter polymer orificesextending through said nosepiece into said channel, said polymer channelmating with said polymer passage formed in said mounting block, and (c)elongated air plates mounted to said mounting block on either side ofsaid die tip by mounting means allowing selection of the position of theair plates with respect to the die tip, defining converging air flowchannels between the die tip and the air plates between the nosepieceand the air plates, and setback of the air plates with respect to theapex of the nosepiece said air flow channels mating with said air flowpassages formed in said mounting block; extruding a molten polymerthrough the orifices of the die tip in use at a meltblowing operatingtemperature; preadjusting said replacement die tip assembly, byassembling said replacement die tip and air plates to said replacementmounting block, such that the replacement die tip assembly exhibitsdesired values for the air-gap and setback; and when it is desired toreplace the die tip assembly in use with the replacement die tipassembly:discontinuing the extrusion of the molten polymer; removing thedie tip assembly in use from the die body; moving the replacement dietip assembly into the position previously occupied by the removed dietip assembly; and securing said replacement die tip assembly to the diebody; and recommencing extruding the polymer through the orifices of thedie tip of said replacement die tip assembly.
 2. The method of claim 1comprising the further step of preheating the replacement die tipassembly to the approximate temperature of the die tip assembly in useprior to said step of moving the replacement die tip assembly into theposition previously occupied by the removed die tip assembly.
 3. Themethod of claim 2 wherein the operating temperature of the die assemblyis between 350 and 900 degrees Fahrenheit, and wherein the replacementdie tip is preheated to within 50 degrees Fahrenheit of the operatingtemperature.
 4. The method of claim 3 wherein the operating temperatureof the die assembly is between about 400 and about 700 degreesFahrenheit and wherein the replacement die tip is preheatedsubstantially to the operating temperature.
 5. The method of claim 4wherein the die assembly is maintained at the operating temperatureduring replacement of the die tip in use with the replacement die tip.6. The method of claim 1 wherein the die body includes an elongatecavity formed therein and each mounting block is complementary incross-section to the cross-section of said cavity, each mounting blockwith the respective die tip mounted thereon being slidable into and outof said cavity.
 7. The method of claim 6 wherein the cross-section ofsaid elongate cavity includes longitudinal support ribs extending alongopposed walls of said cavity, and the cross-section of each mountingblock defines elongate slots accepting said support ribs, whereby saidstep of moving the replacement die tip assembly into the positionpreviously occupied by the removed die tip assembly includes the, stepof sliding said replacement mounting block with the respective die tipmounted thereon into said cavity from one end thereof, such that saidribs fit in said slots of the replacement mounting block to support saidreplacement mounting block.
 8. The method of claim 1, wherein said dieblock defines a cavity for receiving the die tip assembly, and said stepof moving the replacement die tip assembly into the position previouslyoccupied by the removed die tip assembly is performed by lifting thereplacement die tip assembly into said cavity.
 9. A die assembly for ameltblowing machine comprising:(a) an elongated die body defining acentral coathanger-shaped polymer flow passage and air flow passages oneither side thereof, said die body receiving heaters for heating polymerand air flowing in said passages, said die body defining a surface forreceiving a mounting block, and said polymer and air flow passagesterminating at said surface for mating with corresponding passagesdefined by said mounting block; (b) an elongate mounting block adaptedto be received by and secured to said surface of said die body such thatpolymer and air flow passages in said mounting block mate withcorresponding passages in said die body; (c) a die tip adapted to bereceived by and secured to the mounting block and having an outwardlyextending elongate nosepiece of triangular cross-section terminating inan apex, an elongate polymer channel being formed within said nosepiecefor communicating with the polymer flow passage in said mounting block,and a plurality of orifices being formed in at least one row along theapex and communicating with said polymer channel; (d) first and secondair knife plates for being secured to the mounting block on oppositesides of the nosepiece and therewith defining converging air passagesmating with said air flow passages in said mounting block, said airknife plates being spaced from said nosepiece by air-gaps; (e) means foradjustably securing the air knife plates to the mounting block, saidmeans for securing permitting adjustment of the air-gaps and set-back ofthe air knife plates with respect to the apex of the nosepiece; and (f)means for clamping the mounting block with the die tip and air knifeplates secured thereto to the die body and being releasable to permitthe mounting block with the die tip and air knife plates secured theretoto be removed therefrom.
 10. The die assembly of claim 9, wherein saiddie body defines a cavity for receiving said mounting block with the dietip and air knife plates secured thereto, said cavity in said die bodyand said mounting block defining mating shoulders and grooves, wherebysaid mounting block may be supported within said cavity in said die bodyas said mounting block is slid into said cavity from one end thereof.11. The die assembly of claim 9 wherein said means for adjustablysecuring said air knife plates to the mounting block comprises meansdefining a plurality of selectible discrete vertical positions of theair knife plates with respect to the apex of the nosepiece, andselectible spacer members for determining the lateral positions of theair knife plates with respect to the nosepiece.
 12. The die assembly ofclaim 11 wherein said means defining a plurality of selectible discretevertical positions of the air knife plates with respect to the apex ofthe nosepiece comprises a plurality of sets of vertically and laterallyspaced dowel pin receiving holes formed in one of said mounting blockand each of said air plates, and a like plurality of sets of verticallyaligned and laterally spaced dowel pin receiving holes formed in theother of said mounting block and each of said air plates.
 13. The dieassembly of claim 9, wherein said means for clamping the mounting blockwithin the cavity in the die body comprises a plurality of compressionrods spaced along the length of the die body, and adapted to urge themounting block into engagement with the die body.
 14. The die assemblyof claim 13 wherein said compression rods comprise threaded membersthreadingly engaging the die body.
 15. The die assembly of claim 9,wherein said mounting block with the die tip and air knife platessecured thereto is adapted to be lifted toward said die body to contactand be secured to said surface of said die body.
 16. A meltblowing dieassembly with a replaceable die tip assembly, said die assemblycomprising:1) a die body having(a) an elongate cavity formed therein anddefined by opposite inwardly facing side walls and a bottom surface, (b)an elongated coathanger-shaped polymer flow passage discharging throughthe bottom surface, (c) first and second air flow passages on eitherside of said polymer flow passage, and (d) heating means for heatingpolymer and air flowing in said passages; and (2) a die tip assembly,said die tip assembly comprising:(a) an elongate mounting block beingsubstantially coextensive with the cavity and having(i) outwardly facingside walls and an upwardly facing surface, (ii) a downwardly facingsurface, (iii) a polymer flow passage extending from the upwardly facingsurface to the downwardly facing surface, and (iv) first and second airflow passages extending from the upwardly facing surface to thedownwardly facing surface, the mounting block being adapted to bepositioned in the die body cavity (b) an elongate die tip having atriangular, downwardly converging nosepiece terminating in an apex, arow of orifices formed along the apex, an upwardly facing base surface,and a polymer flow passage extending from the base surface to theorifices, (c) air plates for being mounted on the mounting block onopposite sides of the nosepiece and therewith defining converging airpassages, each defining an air gap mating with said first and second airflow passages in said mounting block; and (d) means for securing the airplates to the mounting block, said means for securing comprising meansfor adjusting the relative positions of the air plates with respect tothe nosepiece of the die tip, to permit the presetting of the air gapdefined by each of the air passages defined between said air plates andsaid nosepiece and the setback between the air plates and the apex; (e)means for mounting the die tip on the mounting block, whereby thepolymer flow passages of the mounting block and the die tip are securedin fluid communication; and (f) means for securing the mounting block tothe die body with the die tip and air plates mounted thereon and withthe air and polymer flow passages of the die body and mounting block influid communication, whereby release of the means for securing permitsthe mounting block to be secured to or be removed from the die bodywithout disturbing the adjustment of the air plates with respect to thenosepiece.
 17. The die assembly of claim 16, wherein said cavity in saiddie body and said mounting block define mating shoulders and grooves,whereby said mounting block may be supported within said cavity in saiddie body as said mounting block is slid into said cavity from one endthereof.
 18. The die assembly of claim 16 wherein said means foradjustably securing the air plates to the mounting block comprises meansdefining a plurality of selectible discrete vertical positions of theair plates with respect to the apex of the nosepiece, and selectiblespacer members for determining the lateral positions of the air plateswith respect to the nosepiece.
 19. The die assembly of claim 18, whereinsaid means defining a plurality of selectible discrete verticalpositions of the air plates with respect to the apex of the nosepiececomprises a plurality of sets of vertically and laterally spaced dowelpin receiving holes formed in one of said mounting block and each ofsaid air plates, and a like plurality of sets of vertically aligned andlaterally spaced dowel pin receiving holes formed in the other of saidmounting block and each of said air plates.
 20. The die assembly ofclaim 16, wherein said means for securing the mounting block to the diebody comprises a plurality of compression rods spaced along the lengthof the die body, and adapted to urge the mounting block into engagementwith the die body.
 21. The die assembly of claim 20, wherein saidcompression rods comprise threaded members threadingly engaging the diebody.
 22. The die assembly of claim 16, wherein said mounting block isadapted to be lifted into said cavity in said die body.